Steps to reproduce
1. Admin > Site Administration > Content > Documents and Media
2. Add folder named "folder1"
3. Navigate into 'folder1' and add Basic Document "doc1" - take note of the time when published.
4. Wait 2 minutes, then publish another Basic Document in 'folder1' called "doc2".
5. Move 'doc2' to Recycle Bin.
6. Check last post date value of 'folder1' - found in DLFolder table.
7. Last post date value reflects the posted date of 'doc2' instead of 'doc1'.
Expected outcome: The last post date should show the post date for "doc1".
Actual outcome: The last post date shows the post date for "doc2".
When an FileEntry/FileVersion or Message is deleted, we do not update the last post date for folder/thread to the proper date. We should set it to the date of the last added/updated entry or message.
This may require traversing the entire folder/thread, which should be offloaded to a separate thread to reduce overhead from this kind of metadata.